The present invention relates to an asynchronous transfer mode (ATM) system with a variable cell transmission rate used for transmitting and receiving voice and image signals.
An ATM network for a broadband integrated service digital network (B-ISDN) provides integral system services comprising image, voice, data, etc. as shown in FIG. 1. An ATM line 1 is connected to an ATM transmission station 2, which is connected to a terminal equipment (TE) for providing a subscriber's bus 3 with a telephone, a TV phone, a TV conference, a CATV, data terminal services, etc, Ebrough a network terminal. In the image and voice terminal equipment (TE) a CODEC (encoding and decoding) unit for inputtng and outputtng image and voice signals is provided. As this unit is designed for sending image and voice information as soon as when they are input, it is difficult to restore transmitted information if it is once discarded, causing inconvenience to the time axis process. Thus, a means is required for solving this problem.
An ATM system such as that described above is only in the development stage. No practical embodiment is yet available. However, the following processes have been investigated concerning the discard of a cell, i.e., a unit of transmission information, which is caused to occur in the transmission line and station:
1. correcting information PA1 2. performing progressive encoding to limit the influence of discard to produce the minimum deterioration of images PA1 3. controlling an encoding rate only on the cells discarded on the transmitting point.
However, each process has its problems. In the first process, complete correction becomes impossible with large scale hardware and large volumes of transmitted information. In the second process, it is difficult to realize an algorithm because the discard of cells occurs randomly. In the third process, it is impossible to control all discarded cells in the transmission network.
If a discard of cells is permitted, a burst error may occur. If the data quantity of discarded cells exceeds the contained data of the receiving buffer, an underflowing may occur, causing a discontinuity in the time series process and breaks in images and voice. Furthermore, it is no use adopting an ATM method in a system which can only use a transmission line at a rate that does not cause a discard of cells.